Exactly with the temp. That is a freshwater trick.
Please do not take this the wrong way, but you are saying you know quite a bit, but seem confused about a lot. Reading a few articles really should explain all about the Ich, what treatments work (and don't) and a lot more. The good news is that you are using QT with copper, which works to cure the fish. If you can not keep them in QT for a 6-8 week period, odds are they will go back into the DT which will still have Ich and this vicious cycle will continue. Some fish are strong enough to fight it off and some people just let it run its course and do not QT. They try to keep pristine water conditions and maintain a stress-free environment and hope that it clears itself up. I get that it can work, but I've lost fish to Ich before and that "hope for the best" gameplan just doesn't work for me. IMO
, you are on the right track with what you are doing